1. Field of the Invention
The present invention relates to an apparatus for proportioning reaction mixtures, particularly for the manufacture of hard polyurethane foam The apparatus includes two proportioning cylinders for proportioning each of the components into a mixing head. The pistons of the two proportioning cylinders can be operated alternatingly for filling the cylinders with the component and subsequently ejecting the component from the cylinders.
2. Description of the Prior Art
An apparatus of the type described above for the proportioning of reaction mixtures is known from "Kunststoff-Handbuc" (Plastics Material Manual), Vol.VII, Polyurethane, Carl Hanser Verlag Munich, 1966, Page 138. This known apparatus includes two pairs of proportioning cylinders which are operated by a common mechanical crank mechanism in such a way that during each rotation of the crank one of the proportioning cylinders is filled by taking in the component, while simultaneously the other, already filled cylinder ejects the component.
Accordingly, the known apparatus operates intermittently, i.e., it ejects the two components in a batch-wise manner. The sequence of the proportioning and mixing operations of the apparatus are determined by a constant pressure acting on the supply reservoir for the two components.
However, practical experience has shown that the manner of operation of this known apparatus is unsatisfactory because it is not possible to achieve a constant ejection of the components over the stroke of the oppositely operating pistons of the two proportioning cylinders. This disadvantage of the known proportioning apparatus is due to the operation of the mechanical crank drive which moves the oppositely acting proportioning pistons of the two cooperating proportioning cylinders for each component.
It is, therefore, the primary object of the present invention to provide an apparatus of the above-described type for the proportioning of reaction mixtures, for example, in mixing heads for the production of hard polyurethane foam, which is suitable for a continuous production of a mixture in spite of the intermittent manner of operation.